A storage apparatus is currently being used in data preservation. The storage apparatus is equipped with a plurality of storage devices such as hard disk drives (HDDs) or solid state drives (SSDs) making it possible to use a large capacity storage area. The storage apparatus is connected to a storage control device which performs access control of data writing or data reading for the storage device. The storage apparatus may have a built-in storage control device.
There is a storage device such as, for example, the SSD which has limitations on the number of times of writing to a built-in storage element. In such a storage device, a control process called “wear leveling” designed to extend a product lifetime, may be performed. In the wear leveling, a portion of a physical storage area of the storage device is secured as a spare capacity for wear leveling. The storage device distributes the writing by using the spare capacity in order not to allow the writing to be concentrated on a specific storage area allocated to a user. With this, it may be possible to achieve leveling of the number of times that each storage element undergoes the writing and extend the product lifetime of a storage device. The storage area secured as the spare capacity is made invisible to the user by, for example, firmware of the storage device.
For example, in a storage apparatus equipped with a flash memory, there is a suggestion that a capacity of an alternative area is set by taking the restrictions on the number of times of erasure in the flash memory into account to improve a useful lifetime of the storage area configured by the flash memory. Additionally, there is a suggestion that a logical volume data is completely erased using a chip erasure function of the flash memory when intending to completely erase data of a logical volume such that an increase of the number of times of rewriting data is suppressed and the durable lifetime of a flash memory drive is prolonged compared with a case of executing an erasure method of performing a data overwriting a plurality of times. Furthermore, regarding the flash memory, a method is considered for calculating the number of rewritable times during the lifetime, on the basis of a ratio of the number of valid sectors of a reserved area and the number of sectors that have been and are being used.
Related techniques are disclosed in, for example, Japanese Laid-Open Patent Publication No. 2009-301525, Japanese Laid-Open Patent Publication No. 2008-070935, and Japanese Laid-Open Patent Publication No. 11-073378.
As described above, there is a storage device (e.g., SSD) provided with the spare capacity for wear leveling. In this regard, it may be considered that user capacity may be extended using the spare capacity. However, the size of the spare capacity affects a total amount of writable data (TBW: Total Bytes Written) within a warranty period of the storage device. Specifically, the TBW is increased as the spare capacity becomes larger with respect to the user capacity, and the TBW is decreased as the spare capacity becomes smaller with respect to the user capacity. For that reason, there is a possibility that an accumulated amount of data written into the storage device may exceed the TBW depending on an amount of the writing actually performed onto the storage device. When the accumulated amount of written data exceeds the TBW, there is a problem that the reliability of data preservation in the storage device may be decreased.